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Chemical Elements


isotope of thorium, thoron, Ernest Rutherford, noble gases, atmosphere pressure

Radon, symbol Rn, colorless, odorless radioactive gaseous element that is the heaviest of the noble gases of the periodic table. The atomic number of radon is 86.

Radon was discovered in 1900 by the German chemist Friedrich Ernst Dorn. The colorless gas was named for another element, radium, because radon is produced when radium decays. For several decades radon was believed to be completely chemically inert. Since 1962, however, chemists have been able to make some radon compounds. Radon-222, which is the most abundant isotope of radon, is formed by the radioactive decay of radium-226. Radon-222 has a half-life of 3.8 days, decaying by the emission of alpha particles into an isotope of the element polonium.

Tiny amounts of uranium and radium are naturally present in rock and soil. These elements both produce radon when they decay, and the miniscule amounts of radon gas thus produced can escape into the air. Radon is heavier than air and can concentrate in basements and other unventilated indoor areas built into the ground. Indoor accumulations of the gas pose a serious health hazard. When radon breaks down, it produces charged particles that adhere to dust and other fine matter that can be inhaled by people. Studies indicate that radon may cause up to 21,800 deaths from lung cancer each year in the United States. Several states in the northeastern United States have developed programs to determine whether the gas is present in amounts high enough to pose a risk of lung cancer.

Radon-222 is obtained by passing air through a solution of radium salt and collecting the air and the radon gas that was present in the solution. This isotope can be used in the treatment of malignant tumors. The gas is enclosed in a tube, usually made of glass or gold, called a radon seed, which is inserted in the diseased tissue.

Many other isotopes of radon are known. The isotope of mass 220, discovered in 1899 by Ernest Rutherford, is a product of the radioactive decay of an isotope of thorium and is known as thoron; it has a half-life of 55 seconds. The isotope of mass 219, with a half-life of 4 seconds, is a product of the radioactive decay of an isotope of actinium and is known as actinon. Radon melts at about -71C (about -96F), boils at -62C (-80F), and has a density of 9.73 g/liter at 0C (32F) and 1 atmosphere pressure.

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